1. Field of the Invention
The invention relates in general to read-only memory (ROM). More particularly, the present invention relates to a decoding arrangement for tri-state read-only memory capable of storing more than one bit of data in a single memory cell.
2. Description of Related Art
ROM's (read-only memory) are widely used semiconductor integrated circuits (IC) memory storage devices for digital electronic equipment, in particular, microprocessor-based computer systems, to store predetermined programs. In a conventional semiconductor ROM device, the channel region of a memory cell is selectively implanted with ions to adjust the threshold voltage thereof depending on whether the programmed memory cell is turned on or turned off for representing a logic "1" or a logic "0" in binary, respectively. Conventional ROM IC devices, therefore, store a single binary bit in each of their memory cells. Then, conventional digital circuitry in ROM's employ sense amplifiers to sense the content of an addressed memory cell for "reading." However, the sensed result with respect to each memory cell within the ROM is identified as one of two possible electrical potential states. In other words, the memory content of a ROM memory cell as read is either a logic "1" or a logic "0".
The trend in the semiconductor industry has been to increase the number of memory cells to increase the storage capacity of a semiconductor integrated circuit chip, as well as reduce the size of the semiconductor device itself. Enlarged memory capacity and reduced memory semiconductor die size represent increased functionality and reduced cost, both of which are desirable qualities from a merchandising point of view. Great effort has been expended in the art of semiconductor chip manufacture to reduce the dimensions of semiconductor devices in order to squeeze more memory cells into the same semiconductor die area. This approach, however, is limited by the resolution available according to the current state of the art.
Increasing the number of states to which a memory cell can correspond is a practicable means for overcoming present limitations. Consequently, a tri-state read-only memory is set forth, whose threshold voltage is adjusted through ion implantation with different dosage to be 5V, 2.5V, or 0.8V, to have logic state designated as "1", "-", or "0", respectively However, because the output of the tri-state ROM should still be compatible with other digital systems, it should be able to convert an output tri-state signal into binary code.